1. Field of the Invention
This invention pertains to a membrane switch assembly in which the movement of at least one flexible membrane closes the switch contacts. In membrane switches of the mechanical type, the movement of a flexible membrane simultaneously causes the movement of a conductive member to bridge and close the normally open switch contacts. The membrane movement is typically provided by light finger pressure which moves an internal conductive member through a small gap to close the switch. The present invention includes a further provision for providing tactile and audible feedback to the operator of the switch, while retaining the planar construction and thickness parameters of prior art membrane switches.
2. Description of the Prior Art
In the construction of one type of membrane switch, one or both of the switch contacts is incorporated into an insulative substrate which may comprise a film base or a circuit board panel. The underside of a flexible upper membrane, which overlies the substrate and is spaced apart from the contacts thereon includes a conductive member which may be the other of the switch contacts or a conductive bridge, either of which is adapted to close the contacts upon depression of the flexible membrane. In either case there is a small air gap or space between the contact area of the substrate and the conductive underside of the flexible membrane in the normally open position of the switch. Such constructions are shown, for example, in U.S. Pat. Nos. 3,898,421; 4,365,130 and 4,284,866.
The problem of tactile or audible feedback in the membrane switch has been recognized for some time. The typical membrane switch operation requires a very light force, and a very small deflection to make a contact. Without any type of feedback (visual, audible or tactile) many operators have difficulty sensing switch closure.
One solution to the problem of tactile feel has been the introduction of a dome, either metal or heat formed plastic, which is flexible and which has a certain snap as it is depressed. Some switch designs have incorporated the metal snap domes as active contact members of the switch, acting as shorting pads. Other prior art designs have used an embossed plastic bubble rather than a metal dome. This has been done on the overlay of the membrane switch or on a separate layer between the membrane and the overlay. The plastic bubble concept often produces undesirable tactile characteristics because it does not uniformly deflect over its entire area. Since the bubble does not deform consistently toward the center, an undesirable "teasing" effect may occur. Switch teasing is undesirable because the operator may receive an acceptable tactile feel response, but yet the switch may not close properly. The present invention incorporates a force actuator to reduce any teasing possibilities.
Other methods of producing tactile feel exist but they are difficult to apply in planar construction. Conductive rubber switching can be molded into many different force vs. deflection profiles. However, these molded actuators have package profiles much greater than the typical 0.030"-0.040" membrane switch. Plastic bubbles are often integrated with molded buttons to provide a rigid plunger for actuating the bubble. The button stem acts like a force actuator concentrating the switch force. The drawback to button designs, is also inherent in their non-planar package height. The intent of the present invention is to provide tactile feedback within present membrane thickness parameters.